Electrochemical formation of hydroxide for enhancing carbon dioxide and acid gas uptake by a solution

ABSTRACT

A system is described for forming metal hydroxide from a metal carbonate utilizing a water electrolysis cell having an acid-producing anode and a hydroxyl-producing cathode immersed in a water solution of sufficient ionic content to allow an electric current to pass between the hydroxyl-producing cathode and the acid-producing anode. A metal carbonate, in particular water-insoluble calcium carbonate or magnesium carbonate, is placed in close proximity to the acid-producing anode. A direct current electrical voltage is provided across the acid-producing anode and the hydroxyl-producing cathode sufficient to generate acid at the acid-producing anode and hydroxyl ions at the hydroxyl-producing cathode. The acid dissolves at least part of the metal carbonate into metal and carbonate ions allowing the metal ions to travel toward the hydroxyl-producing cathode and to combine with the hydroxyl ions to form the metal hydroxide. The carbonate ions travel toward the acid-producing anode and form carbonic acid and/or water and carbon dioxide. Among other uses, the metal hydroxide formed can be employed to absorb acid gases such as carbon dioxide from a gas mixture. The invention can also generate hydrogen and oxidative gases such as oxygen or chlorine.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/964,288 for “Electrochemical Formation of Hydroxidefor Enhancing Carbon Dioxide and Acid Gas Uptake by a Solution” filedAug. 9, 2007 by Gregory Hudson Rau. U.S. Provisional Patent ApplicationNo. 60/964,288 for “Electrochemical Formation of Hydroxide for EnhancingCarbon Dioxide and Acid Gas Uptake by a Solution” filed Aug. 9, 2007 byGregory Hudson Rau is incorporated herein by this reference.

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

BACKGROUND

1. Field of Endeavor

The present invention relates to the electrochemical formation ofhydroxide. More particularly the present invention relates toelectrochemical formation of hydroxide from mineral carbonate forenhancing carbon dioxide and acid gas uptake by a solution. Theinvention can also generate hydrogen gas and oxidative gases such asoxygen or chlorine.

2. State of Technology

Due to the climate and environmental effects of excess carbon dioxide(CO₂) in the atmosphere, a variety of methods exist or have beenproposed for pre- or post-emission capture and sequestration of CO₂. Forexample, it is well known that CO₂ will react with hydroxides insolution such that the CO₂ contained in a gas mixture in contact withsuch a solution will be reduced via absorption and reaction within thesolution, and such reactions have industrial applications. More recentlythe use of solutions containing calcium hydroxide (Ca(OH)₂) or sodiumhydroxide (NaOH) have been proposed for large-scale chemical absorptionof air CO₂ using various means of active or passive contacting of air ora gas mixture and the solution. For example, Kheshgi (Kheshgi, H. S.Sequestering atmospheric carbon dioxide by increasing ocean alkalinity.Energy 1995, 20, 915-922) suggested placing calcium oxide (CaO) orCa(OH)₂ in the ocean to effect passive uptake of CO₂ from theatmosphere, largely forming calcium bicarbonate (Ca(HCO₃)₂) in solutionas the CO₂ storage product. Other schemes employ engineered structuresfor the contacting of air with NaOH, forming sodium carbonate (Na₂CO₃)in solution (e.g., Zeman, F. Energy and material balance of CO₂ capturefrom ambient air Environ. Sci. Technol. 2007, 41, 7558-7563; US PatentApplication 2006/0051274 A1; US Patent Application 2006/0093540 A1). Bysubsequently reacting this solution with Ca(OH)₂, calcium carbonate(CaCO₃) is formed and NaOH is regenerated. The CaCO₃ is then calcined athigh temperature to form concentrated CO₂ as the final storage productwhile also forming CaO. The latter is then hydrated to regenerateCa(OH)₂. In this way alkaline hydroxide solutions are recycled andconserved, as opposed to the once-through production and release ofalkalinity in the concept proposed by Kheshgi. However, in both casessignificant quantities of thermal energy are required to either produceor regenerate the hydroxide solutions, especially the calcination ofCaCO₃. This contributes significantly to the cost of either process,plus additional CO₂ is produced if the source of the thermal energy isderived from the combustion of fossil fuels.

Another source of hydroxide is electrochemical salt splitting wherein adissolved salt is split into acid and hydroxide components in thepresence of a charged anode and cathode, respectively. For example asolution containing dissolved sodium chloride (NaCl) can be electrolyzedto form hydrochloric acid (HCl), hypochlorite (ClO⁻), chlorate (ClO₃ ⁻),and/or chlorine gas (Cl₂) at the anode and NaOH at the cathode. Thehydroxide solution can then be removed for subsequent use. Obviously,such electrochemically-produced hydroxide solutions could be used forCO₂ and other acid gas mitigation (e.g., U.S. Pat. Nos. 3,344,050,3,692,649, 3,801,698; US Patent Application 2006/0185985). However,producing hydroxide in quantities sufficient for large scale CO₂ removalcould, in the case of a metal chloride-containing electrolyte, meanmassive co-production of one or more chlorine-containing compounds. Ifthese were not consumed in appropriate ways they would pose asignificant environmental impact.

U.S. Pat. No. 4,337,126 discloses electrolysis of carbonates to producehydroxides. The process is directed to electrolytic production ofhydroxides of alkali metal from alkali metal carbonates contained inwaste streams and naturally occurring carbonate and/or bicarbonatedeposits or ores. Alkali metal carbonates are produced as by-products ina variety of processes which rely on other alkali metal salts or alkalimetal hydroxides as reactants or as treating agents. However, the alkalimetal carbonates used, in particular potassium carbonate, are introducedinto the process in dissolved form and therefore preclude the use ofmore abundant but insoluble alkali metal carbonates such as calciumcarbonate or magnesium carbonate. CO₂ production rather than CO₂mitigation is effected by the invention.

U.S. Pat. No. 5,246,551 discloses electrochemical methods for productionof alkali metal hydroxides without the co-production of chlorine. Alkalimetal hydroxides are manufactured in the United States at the rate ofapproximately 36,500 tons/day, almost entirely by the electrolysis ofaqueous brine solutions, but resulting in the co-production of chlorine.Aqueous solutions of alkali metal carbonates and bicarbonates are usedin the invention, in particular sodium carbonates and bicarbonates,requiring that the alkali metal carbonates and bicarbonates be indissolved form prior to introduction to the system. This precludes theuse of water insoluble alkali metal carbonates such as calcium ormagnesium carbonate that can be much more abundant and less expensivethan water soluble forms for large scale applications. CO₂ is alsoproduced rather than mitigated by this process.

United States Published Patent Application No. 2006/0185985 disclosesremoving carbon dioxide and other pollutants from a gas stream bycontacting with an electrochemically generated hydroxide solution toform metal carbonate and/or bicarbonate. However, in this case the metalsource for the produced hydroxide solution is derived from a solublemetal chloride salt, in particular sodium chloride, requiring theformation of chlorine-containing compounds.

The referenced shortcomings of the preceding methods of hydroxideproduction and CO₂ mitigation are not intended to be exhaustive, butrather are among those that impair or limit their application. A numberof these shortcomings are overcome by the techniques described andclaimed in this disclosure.

SUMMARY

Features and advantages of the present invention will become apparentfrom the following description. Applicants are providing thisdescription, which includes drawings and examples of specificembodiments, to give a broad representation of the invention. Variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this descriptionand by practice of the invention. The scope of the invention is notintended to be limited to the particular forms disclosed and theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The present invention provides an electrochemical system for splittingof metal carbonate, for example calcium carbonate as contained innatural minerals such as limestone, and forming dissolve metalhydroxide. Such hydroxides have wide use in various industrial,chemical, manufacturing, agricultural, aquacultural, and environmentalapplications. For example, such hydroxides can be used as a chemicalfeedstock, used for control or neutralization of acidity, and used foreffecting the flocculation and precipitation of carbonates or othercompounds. In one embodiment the invention employs such hydroxide forabsorbing, neutralizing, and storing carbon dioxide or other acid gases.

In one embodiment the present invention forms metal hydroxide in thecourse of electrolyzing a water solution. This involves the steps ofproviding: a water electrolysis cell having an acid-producing anode anda hydroxyl-producing cathode; a water solution of sufficient ioniccontent to allow an electric current to pass between thehydroxyl-producing cathode and the acid-producing anode submerged in thesolution; placing a metal carbonate in close proximity to theacid-producing anode; a direct current electrical voltage across theacid-producing anode and the hydroxyl-producing cathode sufficient togenerate acid at the acid-producing anode and hydroxyl ions at thehydroxyl-producing cathode; acid dissolution of at least part of themetal carbonate into metal and carbonate ions and allowing the metalions to travel toward the hydroxyl-producing cathode, combining with thehydroxyl ions to form the metal hydroxide, and allowing the carbonateions to travel toward the acid-producing anode to form carbonic acidand/or water and carbon dioxide.

One embodiment of the present invention provides a system for removingcarbon dioxide from the atmosphere or other gas mixtures. Thisembodiment includes the system described above where the metal hydroxidesolution formed is contacted with atmospheric CO₂ or otherCO₂-containing gas mixture in order to absorb and remove some or all CO₂from such gas mixtures. This proceeds via reaction of the excess metalhydroxide with CO₂ or its hydrated form, carbonic acid, to form metalcarbonate or, more preferably for CO₂ absorption purposes, metalbicarbonate. Such metal carbonates or bicarbonates can be used to storeor sequester carbon from the atmosphere or other gas mixture, and/or mayhave other industrial, chemical, or environmental uses. By analogy it isunderstood that the preceding system is relevant for absorbing andneutralizing other gases whose hydrated or unhydrated forms can reactwith metal hydroxide to form metal salts of those gases. Such gasesinclude acid gases such as SO₂, H₂S, and NO₂.

Other embodiments of the invention place the formed metal hydroxidesolution in a separate, natural or artificial gas-solution contactor tofacilitate CO₂ or acid gas removal from a gas mixture. This includes theuse of a natural or artificial water body as both the source of theinitial solution to be electrolyzed as well as the recipient of themetal hydroxide solution produced. In this way the hydroxide once placein the water body is passively contacted with the air or a gas mixtureat the water body's surface thus absorbing CO₂ or other acid gases fromthe atmosphere. Such water bodies include oceans, bays, lagoons, salinelakes and any other body of water whose natural salinity affordselectrolysis of water. Underground saline water may also be used forthis purpose where it is electrolyzed in the presence of metalcarbonate, and the resulting metal hydroxide solution contacted with theatmosphere or other gas mixture using made-made apparatus or natural orartificial water bodies as contactors. Once acid gas absorption has thusbeen achieved by any of the preceding systems, the resulting metalcarbonate or bicarbonate-enriched solution can be left in solution,removed and used for various chemical, industrial, manufacturing, orenvironmental purposes, or discarded.

Hydrogen gas (H₂) is produced as a consequence of the waterelectrolysis, and this gas is collected and can be used as a chemicalfeed stock or fuel. Note that because a CO₂-absorbing solution(hydroxide) is produced simultaneously with the H₂, certain embodimentsof this invention will consume CO₂ while generating H₂. That is, the H₂production process is carbon consumptive or “carbon-negative”, incontrast to the direct or indirect production of significant quantitiesof CO₂ in the present-day commercial manufacture of H₂(“carbon-positive” H₂).

In other embodiments, the H₂ produced is oxidized in a fuel cell orinternal combustion engine, generating electrical, chemical, and/orthermal energy. If, for example, the oxidant is oxygen (O₂), the endproduct is water. If the oxidant is chlorine gas (Cl₂), the end productis hydrochloric acid (HCl). Oxidative gases such as O₂ or Cl₂ can beproduced in the course of the invention's solution electrolysis andthese gases can be used for the preceding H₂ oxidation or for otheruses. Note that as in the case of H₂ generation above, certainembodiments of the invention will allow the production of O₂ or Cl₂ tobe carbon-negative, in contrast to current commercial production methodsof these gases. On the other hand, the reduction or elimination of suchgas emissions from the electrolysis can be accomplished via the use ofgas diffusion electrodes, air electrodes, certain electric currentdensities, or by other methods.

The invention is susceptible to modifications and alternative forms.Specific embodiments are shown by way of example. It is to be understoodthat the invention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of the specification, illustrate specific embodiments of theinvention and, together with the general description of the inventiongiven above, and the detailed description of the specific embodiments,serve to explain the principles of the invention.

FIG. 1 is an illustration of one embodiment of a system constructed inaccordance with the present invention.

FIG. 2 illustrates another embodiment of a system constructed inaccordance with the present invention.

FIG. 3 illustrates another embodiment of a system constructed inaccordance with the present invention.

FIG. 4 illustrates yet another embodiment of a system constructed inaccordance with the present invention.

FIG. 5 is a graph illustrating the results of an experimentaldemonstration of the present invention.

FIG. 6 is flow chart illustrating of one embodiment of a systemconstructed in accordance with the present invention.

FIG. 7 is flow chart illustrating of another embodiment of a systemconstructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, to the following detailed description, and toincorporated materials, detailed information about the invention isprovided including the description of specific embodiments. The detaileddescription serves to explain the principles of the invention. Theinvention is susceptible to modifications and alternative forms. Theinvention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

Referring to the drawings and in particular to FIG. 1 a schematicrepresentation of one embodiment of a system constructed in accordancewith the present invention is shown. FIG. 1 provides a diagrammaticrepresentation of a system of electrochemically generating metalhydroxide in solution via electrolysis of water in the presence of ametal carbonate. “Metal” here is defined as any element in the group IA,IIA, IIIA, IVA, IB, IIB, IIIB, IVB, VB, VIB, VIIB, or VIIIB elements ofthe periodic table where the metal can exist in the forms of metalcarbonate and metal hydroxide. The resulting metal hydroxide is used toabsorb acid gas from the overlying air space, producing metal salts ofthe acid gases. The system also generates hydrogen and other gases. Theinitial system is bounded by a dashed line and designated with thereference numeral 100.

The system 100 provides a method of forming metal hydroxide using thefollowing elements; an acid-producing anode, a hydroxyl-producingcathode, a water solution that contains ions of sufficient quantity toallow electricity to pass through the solution and to split water; adirect current electrical power source 106 of sufficient voltage andcurrent to allow the splitting of water in the solution into hydrogenions (acid) at the cathode and hydroxyl ions at the anode, the hydrogenions being of sufficient concentration to dissolve metal carbonate; amass of metal carbonate in close proximity to the acid-producing anode;impressing an electrical voltage across the anode and the cathodesufficient to generate acid at the anode and hydroxyl ions at thecathode, the acid generated at the anode being of sufficientconcentration to dissolve at least some of the metal carbonate intometal and carbonate ions. The resulting positively charged metal ionsthen travel toward the cathode and combine with the hydroxyl ionsproduced there to form metal hydroxide, while the carbonate ions migratetoward the acid-producing anode to form carbonic acid and/or water andcarbon dioxide.

The system 100 utilizes a porous metal carbonate container 107 tofacilitate the positioning of the metal carbonate near or around theanode 104 under circumstances where the metal carbonate remains in solidor particulate form when placed in the water solution 102. The porosityof the container is such that water molecules and other ions can passthrough part or all of the walls of the container 107, but insolubleparticles will be retained within the container 107. The porouscarbonate container wall can be a membrane, cloth, matrix, grate,filter, sieve, web, screen, or other form of porous material capable ofholding solids. All or part of the top surface of the container 107 maybe left open to facilitate gas collection or release, to allowreplenishment of metal carbonate, and to allow access to the anode 104if positioned inside the container. The container 107 is filled withpieces or particles composed partly or entirely of metal carbonate, inthis example calcium carbonate 108, and the container submerged andpositioned in the water solution 102 such that the solution level 103 isbelow the top of the calcium carbonate mass 108 in the container 107. Acathode 105 is also partially submerged in the water solution 102.

The water solution 102 containing the anode 104 and cathode 105 residesin a larger container 101 capable of holding the water solution 102. Thecontainer 101 could be the ocean or a natural or artificial pond, river,stream, reservoir, vessel, cell, or other natural or artificial bodycapable of holding the water solution 102. The water solution 102 has anionic content that is initially high enough to allow electricity to flowbetween the anode and cathode, the electricity being of sufficientcurrent and voltage to allow the electrochemical splitting of water. Thewater solution 102 either: 1) naturally has sufficient ionconcentration, for example seawater, saline ground water, or salinesurface water, or 2) is artificially made sufficiently saline by theaddition salt ions or by the removal of water (evaporation,desalination, electrodialysis, pressure dialysis, or by other methods).Such salt ions can be metal or non-metal chlorides, sulfates, nitrates,phosphates, carbonates, or any other ions capable of carrying electricalcurrent within the solution.

Electricity is applied from a DC electricity power source 106 throughthe solution 102 via the anode 104 and cathode 105. Water molecules 109in the solution 102 are subsequently split into hydrogen ions 113 andmolecular oxygen 110, with the resulting electrons 114 transferred tothe anode 104. The hydrogen ions 113 then chemically react with thecalcium carbonate 108 to form carbonic acid 112 and calcium ions 111.Being positively charged, the calcium ions 111 migrate towards thenegatively charged cathode 105. The calcium ions 111 combine with thehydroxyl ions 123 formed at the cathode 105 to produce calcium hydroxide118, while the carbonate ions combine with hydrogen ions 113 produced atthe anode 104 to form carbonic acid and/or water and carbon dioxide 112.

In one embodiment the metal hydroxide formed, in this example calciumhydroxide 118, can be used to subsequently react with the carbonic acidand CO₂ 112 and 117 dissolved in the solution, forming primarilydissolved metal bicarbonate 120 when the solution pH is maintained below9, and predominantly metal carbonate when above pH 9. Undercircumstances where metal bicarbonate formation dominates, the carbonicacid 112 formed at the anode 104 is quantitatively insufficient to reactwith all of the metal hydroxides 118 produced in the solution. Theexcess metal hydroxide is then free to react with any additional CO₂dissolved in the solution. As this consumption of dissolved CO₂ reducesthe solution's dissolved CO₂ concentration below the saturation stateallowed by the overlying gas mixture (e.g., air), some CO₂ 115 willdiffuse from the gas into the water solution 102, will be hydrated toform carbonic acid 117, and will react with the metal hydroxide 119 toform metal bicarbonate 120. The net effect is that there will betransfer of CO₂ from the overlying gas 115 to the solution, and the CO₂concentration in the overlying gas 115 is thus reduced. By analogy thismethod can be used to reduce the concentration of other gases that candissolve in water and can react with the metal hydroxide produced 118.Those gases include certain oxides of sulfur and nitrogen such as SO₂and NO₂, as well as H₂S. Potential co-benefits of this system includethe generation of H₂ 104 and O₂ and/or other oxidative gases 110 such asCl₂.

In the system 100, the anode 104 and cathode 105 can be composed ofmaterials that do not chemically react with the solution or itsconstituents, including those composed of graphite, stainless steel,nickel, titanium, tungsten, or platinum. The anode 104 may be composedof a material that preferentially discharges oxygen gas from theelectrolyzed water solution rather than other gases. Additionally, it ispossible to reduce or eliminate gas emission from the electrolyzed watersolution by diverting the hydrogen gas 125 produced at the cathode 105such that it bathes the anode and is oxidized by contact with thechemical constituents produced at the anode 104.

In the system 100, the metal carbonate can be of the type that isinsoluble or sparingly soluble in the solution prior to the applicationof electricity, such as calcium carbonate and magnesium carbonate ascontained in carbonate minerals such as limestone and dolomite. Themetal carbonate can be in particulate, granular, or powdered form so asto allow the water solution and ions to pass through and contact themetal carbonate mass and the anode.

In the system 100, the acid gases such as CO₂ 115 that are absorbed bythe solution 102 can initially reside in a gas stream or parcel, the gasbeing air or other gas from a natural origin, or the gas being from anindustrial, waste, or other anthropogenic source. The gas may be eitherpassively exposed to the solution or may be actively contacted with thesolution by the use of bubbling, stirring, spaying, shaking, mixing, andthe like. The addition of certain enzymes to the solution can also beused to facilitate gas transfer into the solution, for example theaddition of carbonic anhydrase to enhance CO₂ uptake by the solution.

In the system 100, the hydrogen 125, oxygen, chlorine and/or other gases110 generated by system 100 can be collected and can be used forchemical or industrial processes or for other purposes, can be stored,or can be discarded. Note that the simultaneous production of H₂ andoxidative gases with the co-production of a CO₂-consuming metalhydroxide solution allows for the production of H₂ and oxidative gas tobe CO₂-consumptive. This is opposed to conventional H₂ and oxidative gasproduction that is CO₂-emissions-intensive. The metal hydroxide produced118 can be used for purposes other than enhancing acid gas absorption bythe solution 102. Such purposes include use as a chemical feed stock,the neutralization of acidity, and the elevation of solution pH, forexample to effect precipitation of compounds contained in a solution, inparticular metal carbonates and hydroxides. Such carbonates and/orhydroxides can be used, for example, in the production of cement. Theelectricity used in system 100 can be derived from the combustion offossil fuel and/or from non-fossil energy including wind, solar, hydro,wave, tidal, ocean thermal, geothermal, geochemical, biochemical,biomass, or nuclear energy. Non-fossil energy is preferred if maximumnet CO₂ mitigation by the system is desired.

Referring now to FIG. 2, a diagrammatic representation of anotherembodiment of a system constructed in accordance with the presentinvention is shown. This system is designated generally by the referencenumeral 200. The system 200 provides a system for oxidizing the H₂generated from system 100 for the purpose of generating energy andproducing oxidized forms of hydrogen, in particular water or acids suchas hydrochloric acid.

The system 200 includes the unit 201, a system of electrochemicallygenerating excess metal hydroxide in solution via electrolysis of waterin the presence of a metal carbonate, generating hydrogen and othergases. Among other uses, the metal hydroxide produced can be used tochemically absorb acid gases 209 (for example CO₂) from an overlayingheadspace, thus producing metal salts of the acid gases. This unit 201can be a system such as the system 100 described in connection withFIG. 1. Added to unit 201 is a unit 203 for oxidizing the hydrogen 202produced by unit 201 for the purpose of generating electrical,mechanical, and/or thermal energy 207, and of generating oxidizedhydrogen 208. The unit 203 includes a fuel cell or internal combustionengine for oxidizing or combusting the hydrogen 202. The oxidant 204 mayinclude oxygen, chlorine, or other oxidative gas as produced by unit 201and/or as provided from external sources 205. The oxidized hydrogen 208may be in the form of water or acids such as hydrochloric acid, and isremoved from the system and used, stored, or disposed of. Thus system200 generates energy, potentially useful forms of hydrogen compounds,and metal hydroxides which can be employed for absorbing acid gases, orused for other purposes, or which can be stored or discarded.

Referring now to FIG. 3, a diagrammatic representation of anotherembodiment of a system constructed in accordance with the presentinvention is shown. This system is designated by the reference numeral300. The system 300 includes a unit 301 that performs as previouslydescribed by either system 100 or system 200, but where a system isprovided for the periodic or continuous addition to the system of metalcarbonate 302 and water solution 303, and the periodic or continuousremoval of metal salt 304 if produced from the reaction of acid gaseswith the metal hydroxide internally generated in systems 100 or 200. Thewater solution 303 and metal carbonate 302 are added to replenish theions, water, and metal carbonate consumed by systems such as 100 and 200where such consumption and depletion would otherwise negatively affectthe desired performance of the invention. In turn, solid or dissolvedmetal salts 304 (such as previously described calcium bicarbonate,produced by systems such as 100 and 200), formed via aforementioned acidgas contact with the metal hydroxide solution, can be removed fromsystem 300 and may be used for industrial, chemical, manufacturing,agricultural, aquacultural, environmental, or other uses, or may bestored or disposed of.

Referring now to FIG. 4, a diagrammatic representation of anotherembodiment of a system constructed in accordance with the presentinvention is shown. This system is designated generally by the referencenumeral 400. This system is composed of a unit 401 that performs asdescribed for either system 100 or system 200, and where metal carbonate402 and water solution 403 can be added as described by system 300.Unlike the previously described systems, however, the metal hydroxide404 produced, such as calcium hydroxide, is periodically or continuouslyremoved from unit 401 and: 1) placed in a gas-hydroxide contactor 405;2) used for other purposes 406 including industrial, chemical,manufacturing, agricultural, aquacultural, or environmental mitigationpurposes or 3) stored or disposed of. The gas-hydroxide contactor 405 isa natural or artificial structure whose purpose is to facilitate thechemical reaction and absorption of an acid gas 407 such as carbondioxide from the air, waste gas stream, or other gaseous entity. Theresulting solid or dissolved metal salt 408, e.g., dissolved calciumbicarbonate or carbonate, is removed from the contactor 405 and is usedfor industrial, chemical, manufacturing, agricultural, aquacultural, orenvironmental purposes, or is stored or disposed of. In this way system400 allows for the removal of the metal hydroxide from systems 100 or200 and for its use or disposal external from those systems.

Experimental Demonstration

Referring now to FIG. 5, a graph illustrates the results of anexperimental demonstration of the present invention. The FIG. 5 graphshows the time course of seawater pH during 1.5 hrs of electrolysisusing an anode that either was or was not encased in seawater-saturatedCaCO₃ powder, followed by 5 days of solution exposure to ambient air.The corresponding solution [OH⁻]=10^(pH-7.6) at a mean experimenttemperature of 16.5° C. and salinity of 35 ppt. Dashed line denotespre-treatment values.

In the experimental demonstration Applicant placed a 9 cm×1 mm diametergraphite rod anode vertically into a hollow, porous cylindricalcontainer (tea strainer; mean ID=4 cm, height=8 cm) the inside surfaceof which had been lined with a porous paper filter and then filled withreagent grade, powdered CaCO₃. This anode container was then submergedin a glass beaker containing 300 mls of local (Santa Cruz, Calif.)seawater. The anode container was positioned such that the upper surfaceof the CaCO₃ mass was just above the seawater, while the vertical anodepenetrated into the mass such that about 1 cm of the anode was below theseawater level, the submerged part of the anode thus being completelyencased by a seawater-saturated carbonate “paste.” An equivalent, nakedgraphite rod (cathode) was placed vertically into the solution at adistance of about 4 cm from the anode at equivalent seawater depthoutside of the anode container. The initial pH of the seawater solutionwas then measured using a calibrated pH probe (Oakton Model 300).

The anode was then connected to the positive lead and the cathode to thenegative lead of a DC power source providing a measured voltage throughthe cell that ranged from 3.5 to 3.6 V at 6.4 to 7.0 mA. Over 1.5 hrs ofelectricity application the pH of the solution rose to a value of 9.05while electricity was temporarily turned off and after gentle stirringof the seawater to reduce chemical heterogeneity, thus determining truebulk solution pH.

The electricity was then permanently turned off, the electrodes andanode container removed from the seawater, and the solution poured intoa shallow dish (11 cm ID). The pH of the solution was then periodicallymonitored and was observed to return to near its initial value over thecourse of 5 days (FIG. 5). The experiment was repeated without thepresence of CaCO₃ and paper filter (experimental control), with amaximum pH of 8.35 being obtained, followed by a return to pH valuesnear that of the initial seawater (FIG. 5).

It was concluded that the rise in pH observed in both treatments was theconsequent of the reduction and loss from solution of hydrogen at thecathode and the production of mineral hydroxide at the anode. Because ofthe direct, linear relationship between pH and log [OH⁻], the [OH⁻] inthe seawater is calculated to have increased by 25.7 μmoles/L in thecarbonate treatment while it increased by only about 3.4 μmoles/L in thecontrol (FIG. 5). In the latter treatment the OH⁻ generated is presumedto have been balanced by Na⁺ from the splitting of seawater NaCl,whereas Ca²⁺ from the splitting of CaCO₃ is presumed to balance theadditional OH⁻ generated in the carbonate treatment. Evidence of NaClsplitting was indicated by the odor of Cl₂ in both treatments. Thedecrease in pH following the termination of electricity input in bothtreatments is consistent with the excess OH⁻ reacting with CO₂ thatslowly diffused in from the overlying air to form primarily HCO₃ ⁻balanced by the excess Ca²⁺ and/or Na⁺. The greater decline in pH in thecarbonate treatment (FIG. 5) indicates that significantly greaterabsorption of CO₂ was obtained by this treatment than in the control.

Referring now to FIG. 6, a flow chart illustrates one embodiment of asystem constructed in accordance with the present invention. The systemis designated generally by the reference numeral 600. The system 600provides an electrochemical system for splitting of metal carbonate, forexample calcium carbonate as contained in natural minerals such aslimestone, and forming dissolve metal hydroxide. Such hydroxides havewide use in various industrial, chemical, manufacturing, agricultural,aquacultural, and environmental processes. For example, such hydroxidescan be used as a chemical feedstock, used for control or neutralizationof acidity, and used for effecting the flocculation and precipitation ofcarbonates or other compounds. In one embodiment the invention employssuch hydroxide for absorbing, neutralizing, and storing carbon dioxideor other acid gases.

The system 600 provides a method of forming metal hydroxide in thecourse of electrolyzing a water solution. The method includes a numberof individual steps resulting in the production of hydroxides togetherwith H₂ and oxidative gases. The method 600 begins by providing watersolution, metal carbonate, and DC electricity 608 to a waterelectrolysis cell having an acid-producing anode, a hydroxyl-producingcathode, Step 601. In Step 602 the hydroxyl-producing cathode and theacid-producing anode are immersed in the water solution whose ionconcentration is sufficient to provide a current path between thehydroxyl-producing cathode and the acid-producing anode. In Step 603 themetal carbonate is placed near or around the acid-producing anode. InStep 604 DC electricity is impressed across the acid-producing anode andthe hydroxyl-producing cathode of sufficient voltage and current togenerate acidity (H⁺) at the anode and the production of oxidative gases610 such as O₂ and/or Cl₂ (Step 605). The production of acidity near theanode leads to the dissolution of the metal carbonate into metal andcarbonate ions (Step 605). Simultaneously, hydroxyl ions and hydrogengas 612 are produced at the cathode (Step 606). In the cell solution thehydroxyl ions combine (Step 607) with the metal ions to produce metalhydroxide 611. The metal hydroxide, hydrogen, and oxidative gases eitherremain in the cell or are removed for external use, storage, ordisposal.

Referring now to FIG. 7 a flow chart illustrates another embodiment of asystem constructed in accordance with the present invention. The systemis designated generally by the reference numeral 700. The system 700provides a method for removing carbon dioxide from the atmosphere orother gas mixture while also producing either 1) hydrogen and oxidativegases, or 2) energy and oxidized hydrogen compounds. System 700,contains a system 701 that performs as the system shown in FIG. 6. Watersolution 710, metal carbonate 711, and DC electricity 712 are inputs tothe system 701.

In the embodiment now being described the metal hydroxide produced bySystem 701 is contacted with air or other gas mixture (713) tochemically react with and remove some or all of the CO₂ contained in airor gas mixture (Step 702), thus forming metal salts. In this example theCO₂ reacts with the dissolved metal hydroxide to form metal bicarbonate(714) when solution pH is maintained between 6 and 9. The formation ofmetal carbonate will dominate at pH greater than 9. The metal carbonateor bicarbonate formed may remain in System 700 or may be removed foruse, storage, or may be discarded. By analogy it is understood that thepreceding system is relevant for absorbing and neutralizing other gaseswhose hydrated or unhydrated forms can react with metal hydroxide toform metal salts of those gases. Such gases include SO₂, H₂S, and NO₂.

The hydrogen (704) and oxidative gases (703; in this example O₂)produced by System 701 may remain within System 700 or may be removedfor external use, storage, or may be discarded 709. In anotherembodiment the hydrogen is oxidized in a fuel cell or internalcombustion engine (Step 705) within System 700 to produce electrical,mechanical, and/or thermal energy (707) and oxidized hydrogen, in thisexample water (H₂O) 706. The oxidant used in Step 705 may be theoxidative gases (703) produced by System 701 or may be supplied by anexternal source. In this example the oxidant is O₂ produced by System701. System 700 therefore has the ability to couple acid gas mitigation(e.g., CO₂ mitigation) either with hydrogen and oxidative gas production(e.g., O₂ production), or with energy and oxidized hydrogen production(e.g., H₂O production).

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. An apparatus for forming metal hydroxide, comprising: ahydroxyl-producing cathode; an acid-producing anode; a DC electricitysource connected to said hydroxyl-producing cathode and saidacid-producing anode, said power source capable of producing a DCelectric current path between said hydroxyl-producing cathode and saidacid-producing anode; a metal carbonate mass positioned near or aroundsaid acid-producing anode; a water solution into which the saidacid-producing anode and said hydroxyl-producing cathode are at leastpartially submerged, said water solution having a sufficient ionconcentration to allow said DC electric current to pass in said electriccurrent path between said acid-producing anode and saidhydroxyl-producing cathode; and wherein said DC electric current is ofsufficient voltage to electrochemically split water, generating hydroxylions at said hydroxyl-producing cathode and generating acid at saidacid-producing anode, said acid being of sufficient concentration todissolve at least some of said metal carbonate mass into metal ions andcarbonate ions, and wherein said metal ions travel toward saidhydroxyl-producing cathode and form metal hydroxide, and wherein saidcarbonate ions travel toward said acid-producing anode and form carbonicacid or carbon dioxide and water or carbonic acid and carbon dioxide andwater.
 2. The apparatus of claim 1 where said metal carbonate mass is ametal carbonate mass that is insoluble or sparingly soluble in saidwater solution prior to introduction of said electric current.
 3. Theapparatus of claim 2 where said metal carbonate mass is a calciumcarbonate mass or a magnesium carbonate mass, said calcium carbonatemass or a magnesium carbonate mass being manufactured or as contained inor derived from limestone or dolomite.
 4. The apparatus of claim 1further comprising a unit for positioning of said metal carbonate massaround or near said acid-forming anode and wherein said unit forpositioning of said metal carbonate mass around or near saidacid-forming anode is a porous container that holds said metal carbonatemass, said porous container being composed at least partially of amaterial that allows said water solution and said metal ions to passinto and out of the interior of said porous container, said materialbeing a membrane, cloth, matrix, grate, filter, sieve, web, or screen.5. The apparatus of claim 1 where said water splitting results inhydrogen gas being produced at said hydroxyl-producing cathode, andwhere said hydrogen gas is used as a chemical feed stock, is used as afuel in a hydrogen oxidizing apparatus to produce energy and oxidizedhydrogen, or is stored or discarded.
 6. The apparatus of claim 5 wheresaid oxidizing apparatus is a fuel cell or internal combustion engine.7. The apparatus of claim 5 further comprising an oxidant in saidoxidizing apparatus and wherein said oxidant is oxygen or chlorine gas,and wherein the said oxidized hydrogen is water or hydrochloric acid. 8.The apparatus of claim 7 where the said oxidant is produced by the saidacid-producing anode in said apparatus of claim
 1. 9. The apparatus ofclaim 5 where said energy is electrical energy, or mechanical energy, orelectrical energy or a combination of electrical, mechanical, andthermal energy.